Recently LEDs are not only used to indicate a status of some device, to display the time at a digital clock etc. but also for lighting purposes. The latter has been enabled by introducing high output LEDs. Such LEDs can be used e.g. in tail lights, brake lights, turn indicators, side turn indicators and daytime driving lights of a motor vehicle, in pocket lamps or in rooms of a building. They provide various advantages as compared with lamps such as incandescent or luminescent lamps. For example, they can be activated at a higher speed, have a longer service life and take up less space.
When using LED lamps to light a room there may be a need for controlling the intensity of light emitted by LEDs thereof in order to achieve a desired luminance. For LED lamps and other kinds of lamps such control can be accomplished by varying a mean power supplied to the lamps. A device used for this purpose is called a dimmer. Different technologies can be used to implement a dimmer. For example, a potentiometer, a variable transformer, a controllable switching power supply, a phase cut control etc. may be utilized. The phase cut control has a low power loss as compared with the potentiometer and other arrangements based on a variable resistor. Its construction can be smaller and lighter than that of a variable transformer and can be simpler, smaller and less sensitive to breakdowns than that of a controllable switching power supply. For these reasons, phase cut dimmers are widely spread.
There are phase cut-on dimmers in which a leading edge of a waveform of supplied power is cut in order to reduce the mean power. These dimmers ignite (switch on) a certain time period after a zero crossing of the waveform. They are suitable for inductive loads. There are also phase cut-off dimmers in which a trailing edge of the waveform of supplied power is cut in order to reduce the mean power. These dimmers ignite immediately after a zero crossing of the waveform and switch off a certain time period before the next zero crossing. They are suitable for capacitive loads. Further, there are universal dimmers which apply a phase cut-on control for inductive loads and a phase cut-off control for capacitive loads.
LED lamps are much more susceptible to noise on the mains than incandescent lamps or luminescent lamps such as fluorescent lamps. Conventional wall-mount (phase cut) dimmers often introduce extra jitter so that the noise may be visible at the LED lamps. Thus, LED lamps for replacing incandescent or luminescent lamps in mains phase cut dimmer controlled luminaires (i.e. retrofit products) suffer from a visible luminance flicker. Any jitter in an exact location of the phase cut by the dimmer was previously filtered by the slow response of incandescent and luminescent lamps. As the response of LEDs and LED lamps comprising the same is instantaneous, such filtering property is missing. This results in the visible luminance flicker.
FIG. 5 shows an example of typical jitter created by a conventional phase cut dimmer, namely a phase cut-on dimmer. Half of a period of a sine wave representing a mains voltage is depicted. As illustrated in FIG. 5, a time period between a zero crossing of the sine wave and a point in time at which the dimmer ignites is not constant. That is, the dimmer creates jitter, mainly under the influence of mains noise. Consequently, a time period between the point in time at which the dimmer ignites and a next zero crossing of the sine wave is also not constant. As a result, the mean power supplied by the dimmer is not constant. This may cause a visible luminance flicker at LED lamps dimmed by such a dimmer.
A dimmable LED lamp system may comprises the following components: 1) Mains, 2) conventional phase cut dimmer, 3) bridge rectifier, 4) LED driver and 5) LED(s). Such dimmable LED lamp system is likely to produce a visible luminance flicker. The reason for this is that the conventional phase cut dimmer is ‘confused’ by noise and other signals on the mains. As a result, the dimmer creates jitter as shown in FIG. 5. This is usually not visible at incandescent and luminescent lamps because of their slow response behavior, but may be visible when using LED lamps or other lamps having a fast response behavior.
Different ways for reducing a visible luminance flicker are conceivable. FIG. 6 shows a dimmable LED lamp system according to a first approach for overcoming a jitter problem occurring with a conventional phase cut dimmer. The system comprises mains 61, a conventional phase cut dimmer 62, a bridge rectifier 63, an LED driver 64 and LED(s) 65, wherein only a single LED is depicted. In addition, the system includes a low pass filter 66 placed in front of the LED(s) 65. The mains 61 supplies an alternating current (AC) voltage. The mean AC voltage is varied by the dimmer 62 and supplied to the bridge rectifier 63. The bridge rectifier 63 rectifies the AC voltage to a direct current (DC) voltage. The DC voltage is supplied to the LED driver 64. The LED driver 64 drives the LED(s) 65, wherein the low pass filter 66 can filter out high-frequency jitter in an output of the LED driver 64. By this configuration, a visible luminance flicker may be reduced. However, such a low pass filter 66 is bulky (i.e. big) and expensive.
FIG. 7 shows a dimmable LED lamp system according to a second approach for overcoming a jitter problem occurring with a conventional phase cut dimmer. The system comprises mains 71, a conventional phase cut dimmer 72, a bridge rectifier 73, an LED driver 74 and LED(s) 75, wherein only a single LED is depicted. In addition, the system includes a capacitor 76 connected in parallel after the bridge rectifier 73 and a low pass filter 77 interposed between the capacitor 76 and an adjusting input of the LED driver 74. The mains 71 supplies an AC voltage. The mean AC voltage is varied by the dimmer 72 and supplied to the bridge rectifier 73. The bridge rectifier 73 rectifies the AC voltage to a DC voltage. The capacitor 76 is used to stabilize the DC voltage. Thus, the AC voltage is rectified to a stable DC voltage, which is supplied to the LED driver 74. The low pass filter 77 can then be used to filter out a flicker. By this configuration, a visible luminance flicker may be reduced. However, such a capacitor 76 is bulky and expensive.
Since there are no conventionally-dimmable LED lamps on the market, there is very little known on this subject. However, the above described first and second approaches may not be suitable for LED drivers necessitating a space-saving and/or inexpensive construction, such as e.g. LED drivers for retrofit LED lamps intended for replacing incandescent, luminescent or other conventional lamps. This is due to the low pass filter and/or the capacitor being bulky and expensive.